1. Field of the Invention
The present invention relates to a process for producing a thin film, on a substrate, by laser ablation.
2. Description of the Related Art
A process for producing a thin film on a substrate in a vacuum chamber by laser ablation is known, the process including irradiating a laser beam onto a target to cause emission of a substance from the target and allowing the emitted substance to deposit on the substrate to produce a thin film on the substrate.
The laser ablation is advantageous with respect to other methods of producing a thin film in the following points: (1) it provides a high purity thin film only containing impurities in a very small amount because the thin film is solely made of atoms, molecules and ions emitted from the target in a vacuum chamber, (2) an optimum condition for producing a thin film with a desired thickness can be freely selected because many parameters including pressure, substrate temperature, and film growth speed can be separately selected during the film production, and (3) the film growth speed is high and the film production process is very simple to reduce the production cost.
In particular, a process for producing a silicon thin film having good quality by laser ablation is thought to be one of the most promising processes to commercially provide inexpensive high efficiency thin film solar cells, thin film transistors (TFTs) and other electronic and optical devices. The process is being also studied to provide a production process for diamond-like carbon (DLC) thin films, high temperature superconducting thin films, and dielectric thin films.
Laser ablation, however, has a problem in that the emitted substance from a target is not completely dissociated but includes undissociated particles, or clusters, other than dissociated atoms, molecules, or ions and the particles are undesirably introduced into the product thin film. For example, such particles, when present in a silicon thin film for electronic or optical devices, are detrimental to the device performance, and also, oxygen present in the thin film has an adverse effect on the basic properties of a semiconductor such as the electric properties (resistivity, etc.) and the optical properties (light absorption, etc.).
Japanese Unexamined Patent Publication No. 5-279848 proposed to reduce inclusion of the particles by irradiating a laser beam other than that for ablation to the emitted substance from a target to facilitate dissociation of clusters.
The proposal, however, has the drawback that equipment and operating costs are unavoidably raised because a high power laser beam is required to force the clusters to be dissociated. Moreover, it is not practically possible to entirely dissociate the clusters included in the whole volume of the emitted substance. The proposed way is also substantially ineffective in reducing the retained oxygen.